Physical inactivity is associated with an increased incidence of obesity, cardiovascular disease and type II diabetes mellitus. Many children find sedentary activities very reinforcing, and these sedentary behaviors compete with being physically active. We have demonstrated in laboratory and small clinical studies that making access to sedentary behaviors contingent upon being physically active increases the activity level of obese, sedentary children. This research takes advantage of the natural reinforcing effects of sedentary behaviors to motivate children to be active, and has the advantage of simultaneously increasing physical activity and reducing sedentary behavior. The technology that we developed utilized a cycle ergometer that when pedaled above threshold intensity turned on a television or VCR. This technology provides a powerful and precise methodology for increasing activity but has very limited generalization to many physical activities that children engage in. In subsequent laboratory research we demonstrated that when accelerometer counts tallied by exercising on a variety of equipment were the basis for providing access to reinforcing sedentary behaviors, children doubled their physical activity. The present study evaluates the translation of this laboratory technology to clinical research to increase physical activity in obese sedentary children. Overweight/obese boys (n= 45) and girls (n = 45) age 8- 12 years will be randomized to one of three groups; feedback + reinforcement in which children earn access to targeted sedentary behaviors by accumulating activity (counts on an accelerometer), feedback alone in which children monitor their activity but are not reinforced, and a no feedback/no reinforcement control group in which children wear the activity monitor but its output screen is turned off. Our specific aim is to test the hypothesis that subjects receiving the feedback plus reinforcement for physical activity will show greater initial and sustained changes than subjects in the feedback or control group, and that subjects in the feedback group will show greater initial and sustained changes in comparison to subjects in the control group. The primary outcome variable is total physical activity. Secondary outcomes of time in moderate intensity physical activity and sedentary behaviors, body composition physical fitness, and movement competence will be assessed at baseline and then 6 and 12 months after starting treatment. Predictors of change include sex, age and baseline scores of targeted sedentary behavior time and movement competence. This study will yield important information on new ways to increase the physical activity of children.